Processes for preparing relief images which embody photodegradation of a synthetic organic material are known. For example, it is disclosed in U.S. Pat. No. 2,892,712 (Example VII) that a film of formaldehyde polymer coated with a thin layer of omega, omega-dibromoacetophenone was irradiated with ultraviolet light and baked at 105.degree.C. to provide a sheet having a letter text incised into the surface of the film. This system, which relies upon the ability of the dibromoacetophenone to release Br.sup.. radicals which abstract hydrogen atoms from the polymer backbone, could not be used with polyaldehydes other than the difficutly degraded polyformaldehyde since dibromoacetophenone's great reactivity would cause degradation in the dark at normal temperatures. Only by maintaining such a plate at considerably below room temperature could premature degradation be avoided with this sensitizer.
It would be desirable, and it is an object of this invention, to provide a novel method for the preparation of relief printing masters.
A further object is to provide such a method which relies on the ability of polyaldehydes, e.g. poly(acetaldehyde), in admixture with certain photosensitizers to degrade upon exposure to actinic radiation.
Another object is to provide such a process which employs a photosensitizer which does not release halide radicals.
An additional object is to provide such a process in which the master is made by irradiating a film of the polyaldehyde and photosensitizer which film can be stored at ordinary temperatures without substantial degradation prior to exposure.